1. Field of the Invention
The present invention relates to an optical instrument using a Faraday rotator made of Bi-substituted iron garnet single-crystal film formed by LPE (liquid phase epitaxial technique) and having a compensation temperature in order to increase the switching speed of an optical switching device by substantially eliminating hysteresis in the magnetic field dependency of Faraday rotation angle. More specifically, the present invention relates to a magnetooptical polarization plane switch for changing the polarization directions of light by reversing the direction of a magnetic field applied to the Faraday rotator and to an optical switching device using the polarization plane switch. More specifically, the present invention relates to a polarization plane switch in which the magnetization of the Faraday rotator is locally unsaturated and that of the remaining portion of the rotator is saturated when a magnetic field is applied to the rotator, and a light beam passes through the magnetically saturated portion almost vertically to the film surface and to an optical switching device using the switch.
2. Description of the Prior Art
An optical communication system and an optical measuring instrument require an optical switching device for three-dimensionally changing the traveling directions of light. The optical switching device uses a 1.times.1-type optical switching device (optical shutter), a 2.times.2-type optical switching device, or a device fabricated by combining them. A mechanical system, a system using the electrooptical effect or acoustooptical effect, and a magnetooptical system have been proposed as the structure of the optical switching device. Among these system, the magnetooptical system magnetically changes the polarization planes of a light beam by a magnetooptical polarization plane switch set in the system, which has the advantages that it operates at a high speed, it can be downsized, and it has an excellent long-term reliability compared to the mechanical system.
A typical magnetooptical polarization plane switch comprises a yoke made of a magnetic material, a coil provided on the yoke, and a thin-plate Faraday rotator inserted into a gap of the yoke. The Faraday rotator uses a Bi-substituted iron garnet single-crystal film in many cases recently because Bi-substituted iron garnet has a large Faraday rotation factor and therefore it can be formed into a relatively thin structure and moreover the LPE has a high productivity.
The Faraday rotator is inserted into the gap formed on the yoke vertically to the longitudinal direction of the gap. In this case, the Faraday rotator is designed so that part of the rotator protrudes beyond the gap and a light beam passes through the protruding portion. In the case of the prior art, however, the protruding portion of the Faraday rotator is minimized and used by applying a magnetic field in which the magnetization of a garnet single-crystal film is saturated throughout the film surface.
An existing magnetooptical switching device has a switching time of 40 to 250 .mu.sec. As one of the 1.times.1-type switches (optical shutters), the inventors of the present invention proposed an intense-light cutoff apparatus (Japanese Patent Laid-Open No. 6-34894/1994, published Oct. 10, 1994). The apparatus is used to protect an optical part or electronic part by arranging an optical power detector, a delay fiber, and an optical switching device in an optical path in series, quickly turning off the optical switching device when detecting light of excessive intensity, and cutting off the intense light passing through the delay fiber. The cutoff time requested to the above optical switching device is determined by the delay time of an optical fiber. The cutoff time is approx. 1 .mu.sec for the fiber length of 200 m and approx. 10 .mu.sec for the fiber length of 2,000 m. Because the fiber length directly influences the size of the intense-light cutoff apparatus, it is naturally preferable to decrease the fiber length as small as possible. Practically, it is necessary that an optical switching device be capable of cutting off intense light at a cutoff time of 10 .mu.sec or less and with a high reliability.
Since the above-mentioned optical switching device using a Faraday rotator must change magnetic-field directions, it uses not a permanent magnet material but a semi-hard magnetic material as its magnet. Therefore, a very large magnetic field cannot be applied to the device and an LPE film with a small magnetic field Hs necessary for saturation is required. For example, (GdBi).sub.3 (FeAlGa).sub.5 O.sub.12 is the material of a typical LPE film with a small magnetic field Hs necessary for saturation. This material has a small saturation magnetization because it has a compensation temperature (Tcomp) close to an operation temperature (room temperature).
For this type of material, it is generally known that coercive force Hc is inversely proportional to the difference between operating temperature T and compensation temperature Tcomp (Hc oC 1/(T-Tcomp)). It is said that the coercive force Hc is generated due to a defect or strain of a film or the fact that Fe.sup.2+ or Fe.sup.4+ pins domain walls. For example, there is a report that coercive force Hc decreases to 20 Oe as the result of thermally treating a garnet film (Bi.sub.0.8 Tm.sub.2.14 Pb.sub.0.06 Fe.sub.3.1 Ga.sub.1.9 O.sub.12) with a saturation magnetization of 35 G and a coercive force of 100 Oe at room temperature in an oxygen atmosphere for 53 min at 1,050.degree. C. (M. Pardavi-Horvath et al., "Asymmetric switching in high coercivity garnet films", Journal of Magnetism and Magnetic Materials, 104-107, (1992), pp. 433-435).